1. Field of the Invention
The present invention relates to single beam spectrophotometers and, more particularly, to the control of detector hysteresis in a single beam spectrophotometer.
2. Description of the Prior Art
In a spectrophotometer a sample to be measured is positioned in a light beam and light transmitted, scattered, or otherwise passed or radiated by the sample is directed to a light detector, such as a photomultiplier detector (PMT), which generates an output current signal proportional to the intensity of the detected light. In a photomultiplier detector light flux impinging on a photo-emissive cathode is converted into electrons released from the cathode. The electrons are directed at and cascade down a series of dynodes at each of which the number of electrons increases by a process called secondary emission. The multiplied number of electrons is ultimately collected at an anode and is measured as electrical current. A photomultiplier is a very sensitive light detector since the gain or ratio of anode current to cathode current may be as high as 10.sup.8 or more.
A serious problem encountered in photomultiplier detectors is output current variation or hystersis causing corresponding nonlinearity in the detector output current with time. Investigation has uncovered several types of such hysteresis variously termed charge hysteresis, dielectric hysteresis, and leakage hysteresis. Charge hysteresis is defined as a variation in gain of a PMT (at constant cathode potential) caused by electrostatic charges developed on insulators proximate the detector dynodes. Dielectric hysteresis is defined as a variation in polarization at different potentials of any dielectric (insulation) separating conductors of the detector. It is manifested when dynode voltage changes. Leakage hysteresis is defined as the variation in resistance between the dynode and anode elements in a PMT. In addition to hysteresis as aforedescribed such detectors are also subject to fatigue which is defined as a slow loss of the secondary electron emission property of dynodes produced when the dynodes are subject to high levels of electron bombardment.
In any event, the problem of photomultiplier hysteresis has long plagued the field of spectrophotometry. Basically many spectrophotometers employ a photomultiplier to measure the extent to which light is transmitted through or absorbed by a sample material of unknown characteristics. This measurement is compared with corresponding transmittance or absorbance measurement of a reference material of known characteristics. Spectrophotometers for this purpose can be broadly categorized as double beam or single beam in design and operation. In a double beam spectrophotometer the sample material and the reference material are measured in rapid sequence in separate optical paths. The beams of light passing through the sample and through the reference are combined spatially into a common beam and this beam is passed to a single photomultiplier detector. The detector output is demodulated to derive signals indicative of the sample and the reference. Fortunately, in a double beam instrument the time constant for detector hysteresis is usually long relative to the time lapse between the sample and reference measurements. Consequently, detector gain varies little between sample and reference measurements and linearity is preserved.
In a single beam spectrophotometer, on the other hand, the sample and the reference are measured at different times in the same optical path. As a result, a single beam instrument is particularly vulnerable to detector hysteresis since hysteresis alone will cause the detector output current to change with time and light level introducing nonlinearity in the detector output. In this regard the sample and reference measurements are typically made approximately one minute apart. Moreover, in scanning operations, if a sample is measured at different wavelength settings across a wavelength interval, the time between sample and reference measurements can be about five minutes or more. During these time intervals detector hysteresis changes the detector output introducing the aforementioned nonlinearity.
Numerous efforts have been made in photomultiplier design and operation to overcome the hysteresis problem. These efforts have met with some degree of success in that detectors are now commercially available exhibiting a reduction in some hysteresis characteristics.
In spite of these recent advances, however, a heretofore unreported type of photomultiplier hysteresis has been discovered which will be termed "gain hysteresis" hereinafter. In this regard it has been discovered that when a photomultiplier detector is exposed to light with dynode voltage applied the detector exhibits a slow reversible change in gain after such exposure. This effect differs from the previously mentioned charge hysteresis in having a much longer time constant, typically thirty minutes. Moreover it may increase with dynode voltage. It differs from previously mentioned fatigue in that gain may either increase or decrease after light exposure. Moreover it occurs at lower anode currents than usually are present when observing fatigue. It differs from dielectric hysteresis in that it can occur in the absence of any change in dynode voltage. Moreover, the gain hysteresis effect is most apparent after the detector has been left inactive in its "off" or "idle" mode, that is, left with the instrument light source turned off and with dynode voltage removed from the detector. As a result, the gain hysteresis effect is most prominent when the spectrophotometer is first turned "on" in preparation for measuring a sample, i.e., when dynode voltage is applied to the detector and the instrument light source is turned on allowing light therefrom to strike the detector cathode.